application of electrodes in ferro alloy furnaces
TRANSCRIPT
APPLICATION OF ELECTRODES IN FERRO ALLOY FURNACES
Bimal Chatter jee Scientist
National Metallurgical Laboratory Jamshedpur 831 007
Introduction
Ferrp-alloy furnaces are electrothermal in nature where heat is produced by
supplying electric current through the carbonaceous electrodes to • the working space of
the furnace. The properties necessary for the satisfactory performance of an,
electrothermal electrode at elevated temperatures are (1) high electrical conductivity,
(2) slow rate of oxidation (high apparent density), (3) high: mechanical strength
(particles well bonded together i.e. suitable aggregate having thoroughly mixed) and
(4) low heat conductivity. Two types of electrodes are generally in use i.e. carbon
electrodes and graphite. Carbon electrode again are of two kinds i.e. (i) soderberg
electrodes of continuous and self baking type and (II) prebaked carbon electrodes.
Continuous self-baking electrodes are widely used at present.' Carbon electrodes are
very rarely used in the ferro-alloy industry while graphitised electrodes are used in
the production of decarburised ferro-chrome, metallic manganese and crystal silicon in
submerged arc furnaces (SAF). . Graphite electrode find place in submerged arc furnaces'
of such capacities where the electrode diameter is limited to 28". The electrode size
for a submerged arc furnace is selected on the current density for the operation.
Economics has dictated the highest possible current density to keep capital cost to a
minimum. Selecting electrodes by current density may be satisfactory, since it tends
to provide good electrode performance. Ferro-alloy furnaces which are to a large
extent continuously operating units need high current (upto 50000 amperes in three-
phase). For the supply of high current, electrodes of a large' diameter, (20" to 55")
are required. But the difficulties of producing quality electrodes grow' with their
diameter. In the ferro-alloy 'industry, therefore continuous self leaking electrodes
are used exclusively; in the heated up condition they differ very little in their
physical properties from the usual carbon electrodes. The self-baking electrodes are
cheaper than the usual pre-baked carbon electrodes. They can be made on the spot and
the expense in setting up a section to produce the electrode mixture is small compared
with that of a carbon electrode factory. Moreover, no fuel is needed to heat the
electrodes and there is no need of giant cranes to transport and screw the electrodes.
9.01,
The manufacturing process of both the type of electrodes is similar which
comprises of drying, crushing and grinding of the base raw materials and then mixing
with a suitable binder. The paste thus formed, in the case of prebaked electrodes is
extruded in the cylindrical form and baked by calcining at about 1200o -1300°C in
reduced atmosphere. During the process a dense, fully shrunk product is obtained to
give prebaked carbon electrodes. In case of continuous or self baking electrodes, the
green paste is either directly placed into the furnace' or moulded and kept in storage
for use when needed. For graphitised electrodes, the prebaked carbon electrodes after
tar impregnation and rebaking, graphitised by heating in the temperature range 2400-
2600°C C n reducing atmosphere.
The present estimated annual requirement of soderberg paste in India is about
25,000 to 35,000 tonnes. The average consumptions (kgs) of carbon, and soderberg
electrodes for various ferro-alloy production (pertonne) are given below:
Al loy
Carbon electrode Soderberg electrode
Ferro-silicon 20-65 25-80 Ferro-chrome 30-60 40-70 Ferro-manganese 15-30 20-35 Pig iron 9-14 15-20
Production of Electrodes
The production process comprises of (i) selection of raw materials, (ii)
processing and mixing of the raw materials, (iii) forming, (iv) baking
Raw materials:- In the production of electrodes, the selection of raw materials is most
important. The raw materials comprise of (a) dry aggregate and (b) binder.
a) Dry aggregate:- The performance of the electrode is dependent on the proper
selection of dry aggregate.
Ash content within reasonable limits i.e. 6-870 (so as not to affect the
mechanical properties) is tolerable. The following base-materials are generally used
as dry aggregate:
1. Calcined anthracite coal 2. Calcined petroleum coke 3. Pitch coke 4. Metallurgical coke
9.02
Amongst all these materials, calcined anthracite coal is the most desired one as
it gives a paste of good mechanical strength. Generally calcined anthracite coal forms
the coarse fraction of the mix (as it is massive in structure and less porous) and
petroleum coke, pitch' coke or metallurgical cokp form the finer fractions. In India,
as the availability of anthracite being. almost. , nil, calcined petroleum coke is also
used in the coarser fraction although it should not be used for- such fraction because
of its high porosity and thereby providing less strength and inferior properties. At
National Metallurgical Laboratory, a substitute of calcined anthracite coal has been
developed and named as dense carbon aggregate (DCA). It is prepared from raw petroleum
coke, low ash , coke and a medium soft'pitch , binder. It gives properties comparable with
calcined anthracite. The properties of the various base materials used for electrode
making are given in Table I.
Binder:- Binder also plays a very important role in, the production of electrodes. The
binder present in the , mix after carbonizatiOn firmly cements the coke particles *acting
as a bridge between them and finally making the electrode mechanically strong and good
conductor to ,electricity. In general, the coal-tar pitches are preferred as they have
good binding and other properties. The main parameters that control the use of a
binder in an electrode mix are (a) quality, (b) quantity
Quality of a binder mainly depends on: i) Softening pint ii) Coking value iii) Coking reactions of the binder iv) Percentage loss on distillation at low temperature v) Free carbon and soluble resins
Softening point
The softening point of a pitch as determined by well known . tests such as the
Kraemer and Sarnow (K&S) or the Ring and Ball '(R&B) test, is a convenient and suitable
value for expressing the viscosity of the pitch which in turn controls the
workability/flowability of the carbonaceous mix.' Although better quality 'of paste is
obtained by using pitch of higher softening point, medium hard pitch of softening point
R&B 75-90oC is preferred while considering good flowability.
Coking value
Pitches for electrode should have a high yield of coke per unit volume. Pitches
9.03
from coke oven tars are most suitable as they contain less volatile oils and more
carbon than the other types of pitches.
In continuous 0kt:trucks, the pasta (luring its baking in OW, oxpanch: while
loosing its plasticity and finally shrinks. This' Is 'mostly controlled by the reactions
taking place In the binder in the temperature range 300-900°C and making the baked mass porous to a variable extent depending on the quality of the pitch.'
Loss on distillation
Presence of oils in the pitch which are distilled below 360°C control the
swelling of the paste while baking and finally the porosity in the binder coke. A high
temperature pitch containing • less of the volatile oils causes less swelling to the
paste and thereby less porosity.
Free carbon and soluble resins
Free carbon which is the carbonaceous constituent of pitch, Insoluble in •benzene
or toluene also plays a very important role in the performance of the electrode. The
free-carbon is considered as high molecular weight nuclei surrounded by layers of
lower-molecular weight compounds. It is mostly of colloidal dimensions and is partly
soluble in' solvents like pyridine and quinine, capable of forming a colloidal solution.
The portion , of the free-carbon which is soluble in pyridine/quinoline is responsiblp
for giving binding strength .to, the baked product. In addition to free carbon, there
are resins in pitch which are soluble in benzene or toluene but insoluble in light
petroleum and are responsible. for giving wetting and adhesion properties of the binder
towards the grist particles and finally binding the particles together to form a strong
compact. Some properties of the binders are. given in Table
Quantity of Binder
The quantity of binder depends upon the raw material, the granulometry of the
dry aggregate and the type of the electrode for which the mix is used. An extensive
study has crried out at NML on this. In the manufacture of extruded electrode for
carbon/graphite products, around 15 parts by weight of the binder for every 100 parts
of the coke grist gives the maximum density whereas for the soderberg paste, . the binder
quantity varies from 20 to 26 parts for every 100 parts of coke. The higher binder
9.04
content is required for flow properties in the paste while in use. The binder content
should always be upto an optimum value.
Manufacturing Process
Manufacturing process for carbon/graphite electrodes comprises of the following: a) Selection of, proper granulometry of the grist particles b) Mixing of the grist particles with a suitable. binder in a hot mixer
c) Extrusion of the hot mix for carbon/graphite electrode or cast into suitable size for use in soderberg electrode
d) Baking in case of carbon electrodes
e) Baking followed by graphitization
A flow-sheet for the manufacture of electrode paste as well as carbon/graphite
electrodes is shown in Figure 1.
In the selection of proper granulometry with a view to having the highest packing
density, it is important to have various quantities of coarse, medium and fine
fractions of the dry mix in right proportions. In general, the largest particle • in the
medium fraction must have approximately one quarter the diameter of the largest
particle in the coarse range. The finer particle should be chosen such that they fill
up the voids created by the mix between the coarse and medium fractions'.
The dry mix having the proper granulometry and being preheated in the
temperature range 130-145°C is fed into a continuous/batch type sigma blade mixer
through constant weight feeders and the pitch from heated pitch tank, in the molten
condition after passing through a viscosity indicator, a controller and a flow meter,
is injected along with the mix into the same mixer.
Mixing temperature is usually chosen 60-70°C above the R&B softening point and
mixing time of about 30 minutes. In case of paste for soderberg electrode, the paste
is cast into blocks and sent to the customers, who break into small lumps (150 mm to
200 mm) and preheat them before charging into electrodes.
In case of pre-baked carbon/graphite electrodes, the mix being a little
different in consistency but with other* parameters remaining the same, , the mix is
extruded through an extruder capable of applying proper pressure and cut into proper
length. The green electrode's are baked, in case of carbon electrodes and baked,
impregnated rebaked followed by graphitisation for graphitised electrodes.
9.05
Quality Control
For smooth running as well as higher productivity of a ferro-alloy furnace using
soderberg paste/carbon or graphite electrodes proper attention should be given to the
quality of the paste and the electrodes as well. Quality of the paste first of all
depends on the raw materials used in the carbon-mix i.e.the grist material and the
binder. In case of electrodes, in addition to the properties of the mix,. it also
depends on the forming pressure, baking schedule of the green electrodes and finally
the size control. Hence quality control of the raw materials as well as the final
product are an essential part of a paste producing plant. Quality control comprises in
the determination of the properties of the different raw materials required for paste ,
production and finally of the paste also. In case of carbon/graphite electrodes it is
essential to find the properties of the final product and check for the properties
required in a particular case according to the pre-designed specifications.
Green Paste
If raw materials selection is proper and the dry mix having proper granulornetry
is fed into the hot mixing unit, the paste thus produced must have desired properties
too. If a little variation in the properties of the dry aggregate takes place within
limits, by slightly varying the binder percentage, desired flowability of the paste can
be achieved. The flowability tests on sample cylinders are periodically done and 'the
binder adjusted to keep the flowability within limits. It forms the routine control
and is important since in the actual day to day process of manufacture, the only
variable is the binder content. , The apparent density of a good paste should be in the',
range 1.58-1.70 gm/cm3. The green paste in the form of a test electrode is baked at
1000°C to simulate with the electrode paste baked in situ while in operation and some
of the properties normally required for :routine test are given below:
Electrical resistivity 60-90 ohm.ern
Crushing strength 150-250 kg/cm2 Approx. porosity 25-357.
The above . properties are Used for comparison purposes of the batches processed
and indicate the trend quality wise.
Some of the properties of pre-baked carbon/graphite electrode and electrode from
soderberg paste are given in Table 3.
9.06
Use : Soderberg paste
For use of electrode paste in a ferro-alloy furnace, a shell made up of 1 to 2
mm steel sheet is filled with the carbon paste in the form of ltimps average size being
175 mm.
The shell acts as a casing for the electrode paste and protects it against
oxidation. It also provides passage for the electric current from the electrode arm to
the baked section of the electrode and intensifies heat transfer to the upper, unbaked
section of the electrodes. The electrode top should be covered with a lid' to avoid
dust, since it may lead to a subsequent electrode breakage.
A special platform is mounted atop the furnace to facilitate electrode joint
making and filling.
The heat evolving from the furnace softens the paste and enables it to fill the
electrode shell compactly. Temperature for the baking of the electrodes should be such
that at 110-130°C the paste completely softens and form a compact cylindrical shape, at
300-350oC solidification sets in and at 700-8000C solidificatiOn ends with the
evolution of the remaining volatile matter. Distribution of temperature in an
electrode during the electrode baking procedure in large ferro-silicon furnaces is
schematically shown in Figure 2.
Correct operation requires that the electrodes are not baked above the clamps,
for a satisfactory "clamp-electrode" contact is obtained only when electrode clamps in
a softened condition and is "moulded" by then. When the electrode begins to bake abOve
the clamps, the oper'ation of the open disk fan, as well as the electrode 'cylinder and
the gland gasket should be c checked. The cooling of electrodes should be watched as
carefully as that of clamps for an abnormal increase in the temperature causes
premature baling of the paste.
During operation of large soderberg electrode severe difficulties which may
occur are: a) Paste flowing into the bath/furnace b) Uneven current distribution c) Heavy contamination of the' bath
All these problems may be overcome by proper and even distributidri of current,
good granulometry and proper binder with the optimum flowability.
Graphitised electrodes : There are various failures leading to breakage one may
encounter in working with graphitised electrodes: , 1) Poor centering of electrode holders
9.07
2) Poor contact In the nipple Joint whiu ► may result in sparking and local
mitIrtArTmcr
:I) hit urrtt_t c ► is ► gif► g :RI/Mgt-11(J.! .► r:,►► 11 1f 111 ► 'VCI 4)11 I I Ir CO►► 1 ► 111
section of the furnace when it is being cleaned or when attempting to lift an
electrode frozen in the melt etc.
Summary
Use of soderberg paste in the electrodes for ferro-alloy production has been
discussed. The selection of raw materials (viz grist particles and binder), their
desired properties for having a good paste which performs well during operation with
high productivity has also been discussed. Various means of quality control of the raw
materials, paste and baked test electrode and the operation of the electrode also have
been taken care of in this paper.
Use of pre-baked carbon and graphite electrodes which find a little place in the
electrode usage has also been touched upon.
Bibliography
1. Murthy, H P S & Singh R; 'Packing characteristics of coke powders as . a factor in the manufacture of carbon refractories'Journal of Scientific & Industrial Research t 14B, 592-597 11955)
2. Charlette & Bischoferger, G T; 'Binders for carbon electrodes in the aluminium industry'; Vol. 47 (July-Dec.) 1955 Industrial & Engg Chemistry, 47, 1412-1415
3. Amstein, E H et al; The shape of carbon grist particles in the manufacture of carbon electrodes; Industrial Carbon & Graphite; Soc. of Chem. Industry, London, 125-131 (1958)
4. Darney, A;'Pitch binder for carbon electrodes',ibid, 152-161
5. McNeil D and Wood, L J; 'The use of coal tar pitch as an electrode binder',ibid, 162-172
6. Blakeley, T H and Earp, F K; 'Tar and pitches as binders for carbon and graphite', ibid, 173-177
7. Prabhakaram P and Murthy, H P S; `A study of the production of soderberg paste from Indian raw materials'; Symposium on light metal industry in India; National Metallurgical Laboratory, Jamshedpur; 14-17, Feb. 1961, p 105-111
9.08
8. Murthy,H P S arid Chatterjee, 13; 'Certain factors influencin,g the production of dense carbon aggregate from low and high volatile carbonaceous substances';Proceedings, of the seminar • on carbon and carbon products at NPL, Part 1, New Deihl, November 1963
9. Riss, A and Khodorovsky K; Production of ferro-alloys, 1967 Foreign Language Publishing House, Moscow
10. Mantell, C L; Carbon and graphite hand book, Interscience Publishers, 19681968
11. Murthy, H P S, Chatterjee, B and Sarkar, N B; 'Some particulate properties of calcined petroleum coke', NML, Technical Journal, Vol. 18, February 1976, p 6-9
12. Chatterjee Bimal, Sarkar, N B and Murthy H P S; 'A comparative . study of some technological properties of raw materials used in the manufacture of carbon products'; Transactions of the Indian Ceramic Society, Vol. XXXV(4), July-August 1976
13. Murthy, H P S, Chatterjee, B et al; 'Quantity of binder as a factor in the properties of carbon paste'; Indian Journal of Technology, Vol. 15, October 1977, 439-445
14. Vernekar, G V; 'Improved performance of soderberg electrode at VISL, Bhadi-avati' Proceedings of the All India seminar on carbon technology; organised by BALCO, Korba and IIM Korba Chapter, 4-5 March, 1978, 57-64
15. Chatterjee, B et al 'Developments in the production of soder•berg paste for electrodes in ferro-alloy furnaces'; Proceedings of the seminar on Problems and prospects' of ferro-alloy industry in India, organised by NML Jamshedpur and IIM (I&S Division), October 24-26, 1983, 144-151
16. Edneral, F P; The electrometallurgy of . steel and ferro-alloys; Translated by R Hardbottle; Current Book House, Bombay, 1962
TABLE I - PROPERTIES OF DIFFERENT BASE MATERIALS' FOR ELECTRODE PASTE
Ash content, 7.
.Properties. D C.A.
0.4-0.5
Anthracite Coke
6.50-7.50
Calcined Pet. Coke
0.28-0.30
PitchMet Coke
0.215-0.35
Coke
22-25
Sulphur content, %
0.5-1.0 1.50-2.00 0.50-1.00 0.25-0,35 2.00-2.50
True Sp. 2.03-2.05 1.75-1.79 2.01-2.03 1.98-2.00 1.80-1.90 Gr.
Grain Den-
sity g/cm3
1.54-1.56 1.45-1.47 1.49-1.51 1.50-1.52 1.48-1.50
Crushability index
56 5 80 70 65
Electrical resistivity
470-480 1190-1200 430-440 530-550 850-900
Ohm Cm
Microporo- sity, 7.
6-8 3-4 10-12 4-6 12-14
TABLE 2, - PROPERTIES OF BINDERS FOR ELECTRODE PASTE
Material Softening pt °C Free Carbon Coking Value 7. (K&S) 7.
Coal tar 15-20 7-25 20-30
Pitch
Soft 50-70 15-25 25-35
Medium 70-90 25-35 35-45
Hard 90-140 30-50 40-60
9.10
TABLE 3 - PROPERTIES OF SODERBERG ELECTRODE, CARBON AND GRAPHITISED ELECTRODES
Properties Soderberg Carbon Graphitised electrode electrode electrode (Cal.Fcl.coke based)
Approximate 25-35 porosity, 7.
B.D. , g/cm3 1.45-1.50
Ash 4.25-0.30 content, 7. (4-0*
Sp. elect. 60-90 resistance Ohm. Cm
Oxidation 425-475
temp °C
C.C.S. 150-250
kg/cm2
M.O.R. 70-80
kg/cm2
20-25 22-25
1.45-1.60
1.50-1.70
4-8
0.1-0.5
35-55 8-12
400-500 600
200-450 200-350
80-175 80-100
*Anthracite based
I
L
' ralcintatIon
S creening
-f Mir
vritlitiny
EL_
Craplul1s/11m
I ifachining
Pouring Tinishea pro-arias storage
Proportioning ----
Mixing
Graphite storage
Nader storage
]
Preparation , of binder
Carbon materials storage
T
Primary crushing
Cruibiny
Ramming
• Pan mill processing
Pressing
flaming
Fig. 1 Flowsheet for manufacturing electrode
1
9.12
5
Fig. 2 Piagram or temperature zones in a self-baking electrode
9.13